Semiconductor device and manufacturing method thereof

ABSTRACT

A semiconductor device includes a substrate, a first semiconductor fin and a gate stack. The first semiconductor fin is over the substrate and includes a first germanium-containing layer and a second germanium-containing layer over the first germanium-containing layer. The first germanium-containing layer has a germanium atomic percentage higher than a germanium atomic percentage of the second germanium-containing layer. The gate stack is across the first semiconductor fin.

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 62/750,776, filed Oct. 25, 2018, which is herein incorporatedby reference in its entirety.

BACKGROUND

As the semiconductor industry has progressed into nanometer technologyprocess nodes in pursuit of higher device density, higher performance,and lower costs, challenges from both fabrication and design issues haveresulted in the development of three-dimensional designs, such as a finfield effect transistor (FinFET). A typical FinFET is fabricated with athin vertical “fin” (or fin structure) extending from a substrate formedby, for example, etching away a portion of a silicon layer of thesubstrate. The channel of the FinFET is formed in this vertical fin. Agate is provided over (e.g., wrapping) the fin. Having a gate on bothsides of the channel allows gate control of the channel from both sides.

However, there are challenges to implementation of suitable channelmaterials on a single substrate and processes in complementarymetal-oxide-semiconductor (CMOS) fabrication, thereby degrading thedevice performance.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 32 illustrate the cross-sectional views and perspectiveviews of intermediate stages in the formation of Fin Field-EffectTransistors (FinFETs) in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figs. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

A method of forming Fin Field-Effect Transistors (FinFETs) on a hybridsubstrate and the resulting structures are provided in accordance withvarious exemplary embodiments. The intermediate stages of forming thehybrid substrate and the FinFETs are illustrated in accordance with someembodiments. Some variations of some embodiments are discussed.Throughout the various views and illustrative embodiments, likereference numbers are used to designate like elements.

The fins may be patterned by any suitable method. For example, the finsmay be patterned using one or more photolithography processes, includingdouble-patterning or multi-patterning processes. Generally,double-patterning or multi-patterning processes combine photolithographyand self-aligned processes, allowing patterns to be created that have,for example, pitches smaller than what is otherwise obtainable using asingle, direct photolithography process. For example, in one embodiment,a sacrificial layer is formed over a substrate and patterned using aphotolithography process. Spacers are formed alongside the patternedsacrificial layer using a self-aligned process. The sacrificial layer isthen removed, and the remaining spacers may then be used to pattern thefins.

FIGS. 1 through 16 illustrate the cross-sectional views and perspectiveview of intermediate stages in the formation of the hybrid substrate andthe FinFETs in accordance with some embodiments of the presentdisclosure.

Referring to FIG. 1, a hybrid substrate 20 is provided. The hybridsubstrate 20 includes a crystalline silicon layer 22, a dielectric layer24 over the silicon layer 22, and a crystalline silicon layer 26 overthe dielectric layer 24. The dielectric layer 24 may be formed ofsilicon oxide or other dielectric materials such as silicon nitride,silicon carbide, etc. The silicon layer 26 is bonded to the dielectriclayer 24. The hybrid substrate 20 includes p-type well regions 100 andan n-type well region 200 between the p-type well regions 100. Thep-type well regions 100 and the n-type well region 200 are formed, forexample, through implantations. The p-type well regions 100 can beequivalently referred to as n-type device regions, and the n-type wellregion 200 can be equivalently referred to as a p-type device region insome embodiments.

The silicon layer 22 is a (100) substrate having a (100) surfaceorientation, with the top surface of the silicon layer 22 in the (100)plane of silicon. In accordance with some embodiments of the presentdisclosure, the silicon layer 26 is a (100) R45 layer, which is formedby rotating a (100) substrate by from about 40 degrees to about 50degrees before cutting and bonding to dielectric layer 24. As a result,the top surface of the (100) R45 layer has a (100) R45 surfaceorientation, and the sidewalls of the resulting fins (discussedreferring to FIG. 7) are also on the (100) plane of silicon.

Referring to FIG. 2, an epitaxy is performed to grow a silicon layer 28on the silicon layer 26. Depending on the orientation of the siliconlayer 26, the silicon layer 28 may be a (100) R45 layer. The siliconlayer 28 may be free from germanium. Furthermore, the silicon layer 28may be intrinsic, with no p-type and n-type impurity doped in theepitaxy. In accordance with alternative embodiments, the silicon layer28 is in-situ doped with a p-type impurity during the epitaxy. Thethickness of silicon layer 28 may be close the fin height of theresulting FinFETs.

FIG. 3 illustrates the recessing of the silicon layer 28 and the hybridsubstrate 20 in the p-type device region 200, and the recessing is notperformed in the n-type device regions 100. Recess 35 is thus formedbetween the n-type device regions 100. In accordance with someembodiments of the present disclosure, to perform the recessing, acapping layer 30 is formed first as a blanket planar layer, for example,through thermal oxidation or deposition. The capping layer 30 may beformed of silicon oxide or other dielectric materials such as siliconnitride, silicon carbide, or silicon oxynitride. The recessing is thenperformed. During the recessing, the capping layer 30, the silicon layer28, and the silicon layer 26 are etched-through, exposing the topsurface of the underlying dielectric layer 24, which is then etched. Thesilicon layer 22, which has the (100) surface plane, is thus exposed.

Next, a spacer layer is deposited, followed by an anisotropic etch toremove the horizontal portions of the spacer layer, so that spacers 32are formed on sidewalls of the recess 35. In accordance with someembodiments of the present disclosure, the spacers 32 are formed of adielectric material such as silicon oxide (SiO₂), silicon nitride, orthe like. Due to the different materials for forming the capping layer30 and the spacers 32, the capping layer 30 remains after the formationof the spacers 32. Hence, both the sidewalls and the top surface ofsilicon layers 26 and 28 in the n-type device regions 100 are masked.

After the spacers 32 are formed, a first epitaxy process and a secondepitaxy process are performed in sequence to form first and secondepitaxy layers 34 and 36, and the resulting structure is shown in FIGS.4A and 4B. FIG. 4A illustrates a perspective view of the resultingstructure, and FIG. 4B illustrates a cross-sectional view along line B-Bin FIG. 4A. The first epitaxy layer 34 is formed over the silicon layer22, and the second epitaxy layer 36 is formed over the epitaxy layer 34.In accordance with some embodiments of the present disclosure, the firstand second epitaxy layers 34 and 36 are formed of a high-mobilitysemiconductor material such as silicon germanium, germanium (with nosilicon), III-V compound semiconductor such as GaAs, InP, GaN, InGaAs,InAlAs, GaSb, AlSb, AlAs, AlP, GaP, combinations thereof, ormulti-layers thereof. In some embodiments, the first and second epitaxylayers 34 and 36 may include same materials, for example, SiGe.

The first and second epitaxy processes are selective epitaxy processes.In greater detail, an etching gas such as HCl is added in the processgases, so that the first epitaxy layer 34 is grown from the top surfaceof silicon layer 22, and not from dielectric materials such as thecapping layer 30 and the spacers 32. Similarly, the second epitaxy layer36 is grown from the first epitaxy layer 34, and not from the dielectricmaterials. The spacers 32 mask the sidewalls of silicon layers 26 and28, so that the epitaxy is achieved from a single surface (the topsurface of silicon layer 22), and hence defects caused by growing fromdifferent surfaces are avoided.

The first epitaxy layer 34 is formed one or more epitaxy or epitaxial(epi) processes, such that semiconductor materials can be formed in acrystalline state on the silicon layer 22. In some embodiments, the oneor more epitaxy processes of forming the first epitaxy layer 34 includeCVD (chemical vapor deposition) deposition techniques (e.g., vapor-phaseepitaxy (VPE) and/or ultra-high vacuum CVD (UHV-CVD)), molecular beamepitaxy, and/or other suitable processes. The epitaxy process may usegaseous and/or liquid precursors, which interact with the composition ofthe underlying semiconductor materials. In some embodiments where thefirst epitaxy layer 34 includes SiGe, examples of silicon-containinggases include silane (SiH₄) or the like, and examples ofgermanium-containing gases include germane (GeH₄) or the like.

It is observed that due to different growth rates on different surfaceplanes of silicon germanium, facets may be formed. For example, thegrowth rate on surfaces having (111) surface orientations (referred toas (111) planes) is lower than that on other planes, such as (110) and(100) planes. Accordingly, the facets FA1, which have the (111) surfaceorientation (in other words, on (111) planes), have the lowest growthrate, which other planes have higher growth rates. In the beginning ofthe epitaxial growth of the SiGe layer 34, facets FA1 may not be formed.However, with the proceeding of the epitaxial growth, due to thedifference in growth rates, facets FA1 are gradually formed. Therefore,in some embodiments where the first epitaxy layer 34 includes silicongermanium, the first epitaxy layer 34 has a first portion 34 a grown onthe (100) plane of the underlying silicon layer 22 and a second portion34 b grown on the (111) plane of the (111) facets FA1. In other words,the first and second portions 34 a and 34 b are interfaced at the (111)facets.

Process conditions are selected such that at least the initially grownSiGe portion 34 a can achieve a desired germanium concentration (i.e.,germanium atomic percentage). However, due to the nature of SiGe epitaxygrowth, the germanium concentration of SiGe grown on the (111) planewill be higher than that grown on the (100) plane. As a result, thesecond portion 34 b has a higher germanium concentration than that ofthe first portion 34 a, thus resulting in unwantedly increased germaniumconcentration in the second portion 34 b. In other words, the germaniumconcentration of the second portion 34 b is higher than the germaniumconcentration of the first portion 34 a. For example, the germaniumconcentration of the second portion 34 b and the germanium concentrationof the first portion 34 a have a difference greater than about 5%. Forexample, the germanium concentration of the first portion 34 a is in arange from about 20% to about 35%, and the germanium concentration ofthe second portion 34 b is in a range from about 25% to about 40%. Ifthe epitaxy process of the first SiGe layer 34 continues until therecess 35 is filled with the second portion 34 b, the resulting fins(See FIG. 7) would include a large region (i.e., large second portion 34b) having unwantedly increased germanium concentration and a smallregion (i.e., small first portion 34 a compared to the large secondportion 34 b) having the desired germanium concentration. For example, avolume percentage of the portion having unwantedly increased germaniumconcentration in the resulting fin might be from a range about 60% toabout 66%, which in turn aggravates non-uniformity of germaniumconcentration in the resulting fins.

As a result, epitaxy growth of the first SiGe layer 34 stops as long asthe epitaxy growth on the (111) plane occurs. Afterwards, processconditions for the SiGe epitaxy process are changed to epitaxially growthe second SiGe layer 36 on the first SiGe layer 34. Less germanium isintroduced in the formation of the second SiGe layer 36 than in theformation of the first SiGe layer 34. In this way, the resulting secondSiGe layer 36 has a lower germanium concentration than the secondportion 34 b of the first SiGe layer 34. As a result, the non-uniformityof the germanium concentration in the resulting fins is mitigated inspite of the unwantedly high germanium concentration occurs in thesecond portion 34 b of the first SiGe layer 34. For example, a volumepercentage of the portion having unwantedly increased germaniumconcentration (i.e., second portion 34 b of the first SiGe layer 34) inthe resulting fin might be from a range about 4% to about 8%, which inturn significantly reduce non-uniformity of germanium concentration inthe resulting fins.

Similar to the first SiGe layer 34, the second SiGe layer 36 has a firstportion 36 a grown on the (100) plane of the underlying first portion 34a of the first SiGe layer 34 and a second portion 36 b grown on the(111) plane of second portion 34 b of the first SiGe layer 34. Due tothe nature of SiGe epitaxy growth, the germanium concentration of SiGegrown on the (111) plane will be higher than that grown on the (100)plane. As a result, the second portion 36 b of the second SiGe layer 36has a higher germanium concentration than that of the first portion 36 aof the second SiGe layer 36. Therefore, process conditions of formingthe second SiGe layer 36 are selected such that the first portion 36 aof the second SiGe layer 36 has a germanium concentration lower than adesired germanium concentration (i.e., germanium concentration of thefirst portion 34 a of the first SiGe layer 34), so as to reduce agermanium concentration difference between the second portion 36 b ofthe second SiGe layer 36 and the first portion 34 a of the first SiGelayer 34. In some embodiments, the germanium concentration of the firstportion 36 a of the second SiGe layer 36 is lower than the germaniumconcentration of the first portion 34 a of the first SiGe layer 34 by atleast about 5%. In this way, the germanium concentration of the secondportion 36 b of the second SiGe layer 36 may be substantially the sameas the germanium concentration of the first portion 34 a of the firstSiGe layer 34, thus resulting in improved uniformity of germaniumconcentration in the resulting fins. For example, the germaniumconcentration of the first portion 36 a of the second SiGe layer 36 isin range from about 15% to about 30%, and the germanium concentration ofthe second portion 36 b of the second SiGe layer 36 is in a range fromabout 20% to about 35%.

The formation of the second SiGe layer 36 may be performed in-situ withthe formation of the first SiGe layer 34, which means that first andsecond SiGe layers 34 and 36 are formed in a same process chamber withno vacuum break occurring therebetween. In some exemplary embodiments,the partial pressures, hence flow rates of the germanium-containinggases such as GeH₄ are reduced to reduce the germanium concentration inthe second SiGe layer 36. In some embodiments, the germanium-containinggas partial pressure difference between growing the first SiGe layer 34and growing the second SiGe layer 36 is in a range from about 30 mtorrto about 150 mtorr. If the GeH₄ partial pressure difference betweengrowing the first SiGe layer 34 and growing the second SiGe layer 36 isgreater than about 150 mtorr, the germanium concentration differencebetween the second portion 36 b of the second SiGe layer 36 and thefirst portion 34 a of the first SiGe layer 34 might be unsatisfactoryfor improving the germanium concentration uniformity in the resultingfins. If the GeH₄ partial pressure difference between growing the firstSiGe layer 34 and growing the second SiGe layer 36 is less than about 30mtorr, the germanium concentration difference between the second portion36 b of the second SiGe layer 36 and the first portion 34 a of the firstSiGe layer 34 might be unsatisfactory for improving the germaniumconcentration uniformity in the resulting fins. In some embodiments, thefirst and second epitaxy layers 34 and 36 are in combination referred toas a multi-layer epitaxy structure 37.

Moreover, defects such as misfit dislocations might occur at theinterface between the silicon layer 22 and the SiGe structure 37, if thethickness of the SiGe structure 37 is greater than a critical thickness.Stated differently, the critical thickness is a thickness at whichmisfit dislocations may start to appear. However, the critical thicknessassociated with the misfit dislocations is in positive correlation ofthe average germanium concentration in the SiGe structure 37. Becausethe SiGe structure 37 includes reduced average germanium concentrationas discussed above, the critical thickness of the SiGe structure 37 canbe increased, which in turn will result in reduced misfit dislocations.In some embodiments, the first SiGe layer 34 has a thickness T1 in arange from about 50 nm to about 70 nm. If the thickness T1 of the firstSiGe layer 34 is greater than about 85 nm, misfit dislocations might beincreased. In some embodiments, the second SiGe layer 36 has a thicknessT2 in a range from about 50 nm to about 100 nm. If the thickness T2 ofthe second SiGe layer 36 is greater than about 150 nm, misfitdislocations might be increased.

After the epitaxy growth of the SiGe layers 34 and 36, a planarizationstep such as Chemical Mechanical Polish (CMP) or mechanical grinding isperformed to level the top surface of the SiGe structure 37, as shown inFIGS. 5A and 5B. FIG. 5A illustrates a perspective view of the resultingstructure, and FIG. 5B illustrates a cross-sectional view along line B-Bin FIG. 4A. In accordance with some embodiments of the presentdisclosure, the planarization is performed using silicon layer 28 as astop layer. In accordance with alternative embodiments of the presentdisclosure, the planarization is performed using capping layer 30 as astop layer, followed by an etching process to remove capping layer 30.In some embodiments, after the CMP process, the first portion 36 a ofthe second SiGe layer 36 is absent from the remaining SiGe structure 37.In alternative embodiments, after the CMP process, the first portion 36a of the second SiGe layer 36 remains in the SiGe structure 37.

Referring to FIG. 6, a protection layer 38 is formed. In accordance withsome embodiments of the present disclosure, the protection layer 38 isformed of silicon, and is deposited on the top surface the structureshown in FIG. 5A. The protection layer 38 is also free from germanium insome embodiments. The deposition may be achieved through an epitaxyprocess, so that the silicon layer is a crystalline layer. In accordancewith alternative embodiments of the present disclosure, the siliconlayer 38 is a polysilicon layer.

The following steps as shown in FIGS. 6 and 7 illustrate the formationof semiconductor strips. The strips may be patterned by any suitablemethod. For example, the strips may be patterned using one or morephotolithography processes, including double-patterning ormulti-patterning processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, allowing patterns to be created that have, for example,pitches smaller than what is otherwise obtainable using a single, directphotolithography process. For example, in one embodiment, a sacrificiallayer is formed over a substrate and patterned using a photolithographyprocess. Spacers are formed alongside the patterned sacrificial layerusing a self-aligned process. The sacrificial layer is then removed, andthe remaining spacers, or mandrels, may then be used to pattern thestrips.

In accordance with some exemplary embodiments as shown in FIG. 6, masklayer(s) are deposited over the protection layer 38, and are thenpatterned to form masks 40, which are used as etching mask for formingsemiconductor strips. In accordance with some embodiments of the presentdisclosure, the masks 40 include a plurality of layers formed ofdifferent materials. For example, the masks 40 may include layers 40Aformed of silicon oxide, and layers 40B over the respective layers 40,with layers 40B formed of silicon nitride. In the formation of the masks40, the protection layer 38 protects the underlying SiGe structure 37from being oxidized, for example, due to the elevated temperatureadopted in the deposition of the mask layers. Silicon germanium is proneto oxidation, and also has an oxidation rate significantly higher thanthe oxidation rate of silicon. Accordingly, by forming the protectionlayer 38, the SiGe structure 37 is protected from the undesirableoxidation.

Referring to FIG. 7, an etching process is performed to etch the hybridsubstrate 20 and the epitaxy layers, so that strips 142 and 242 areformed in n-type device regions 100 and p-type device region 200,respectively. Recesses 44 are formed to separate strips 142 and 242.Strips 142 include portions 122, 124, and 127. Strip portions 122 arethe remaining portions of the patterned silicon layer 22. Strip portions124 are the remaining portions of dielectric layer 24 (FIG. 6). Stripportions 127 are the remaining portions of silicon layers 26 and 28(FIG. 6). In accordance with some embodiments of the present disclosure,silicon layers 26 and 28 are (100) R45 layers. Strips 242 includeportions 222, 234 a and 234 b and 236. Strip portions 222 are theremaining portions of the patterned silicon layer 22. Strip portions 234a are remaining portions of first portion 34 a of the first SiGe layer34 (FIG. 6). Strip portions 234 b are remaining portions of the secondportion 34 b of the first SiGe layer 34 (FIG. 6). Strip portions 236 arethe remaining portions of the second portion 36 b of the second SiGelayer 36 (FIG. 6). Accordingly, strip portions 234 a and 236 are SiGeportions and have substantially the same germanium concentration, asdiscussed previously with respect to FIGS. 4A and 4B. Strip portions 234b are SiGe portions having higher germanium concentrations than stripportions 234 a and 236, as discussed previously with respect to FIGS. 4Aand 4B. In some embodiments, a germanium concentration differencebetween the strip portions 234 a and 234 b is greater than a germaniumconcentration difference between the strip portions 234 a and 236, and agermanium concentration difference between the strip portions 234 b and236 is greater than the germanium concentration difference between thestrip portions 234 a and 236 as well.

FIG. 8 illustrates the formation of a liner 48, which is used to maskthe sidewalls of SiGe strip portions 234 a, 234 b and 236 fromoxidation. In accordance with some embodiments of the presentdisclosure, the liner 48 is formed of silicon, and is free from orsubstantially free from germanium. Furthermore, liner 48 may be freefrom oxygen and nitrogen, and hence does not include silicon oxide andsilicon nitride. The formation of liner 48 may be performed using aconformal deposition method such as Atomic Layer Deposition (ALD) orChemical Vapor Deposition (CVD).

FIG. 9 illustrates the formation of STI (Shallow Trench Isolation)regions 50, which includes dielectric liners 52 and dielectric regions54 over dielectric liners 52. In accordance with some embodiments of thepresent disclosure, a conformal deposition method is used to deposit aconformal dielectric liner 52 on the exposed surfaces of the structureshown in FIG. 8. For example, dielectric liners 52 may be formed usingALD or CVD. Next, the remaining portions of recesses 44 (FIG. 8) arefilled with a dielectric material 54. The dielectric material 54 may beformed using Flowable Chemical Vapor Deposition (FCVD), spin-on coating,or the like. In accordance with some embodiments in which FCVD is used,a silicon- and nitrogen-containing precursor (for example, trisilylamine(TSA) or disilylamine (DSA)) is used, and hence the resulting dielectricmaterial is flowable (jelly-like). In accordance with alternativeembodiments of the present disclosure, the flowable dielectric materialis formed using an alkylamino silane based precursor. During thedeposition, plasma is turned on to activate the gaseous precursors forforming the flowable oxide.

In the formation of the dielectric liners 52 and the dielectric regions54, the temperature of the formation process may be elevated, which islikely to cause the oxidation of the SiGe strip portions 234 a, 234 band 236 if the strip portions 234 a, 234 b and 236 are exposed. Theliner 48 (FIG. 8) thus protects the SiGe strip portions 234 a, 234 b and236 from the oxidation. As a result, the liner 48, (or at least theportions of the liner 48 contacting strip portions 124, 127, 234 a, 234b and 236) may be oxidized during the formation of dielectric liners 52and dielectric regions 54, and hence is converted into a silicon oxidelayer.

Next, a planarization such as CMP or mechanical grinding is performed onthe dielectric regions 54 and the dielectric liners 52. Theplanarization may be performed using masks 40 (FIG. 8) as a stop layer.Next, the masks 40 are removed, followed by recessing the dielectricregions 54 and the dielectric liners 52. The resulting structure isshown in FIG. 9. The remaining portions of the dielectric regions 54 andthe dielectric liners 52 are referred to as the STI regions 50. Inaccordance with some embodiments of the present disclosure, therecessing is performed until the recessed STI regions 50 have their topsurfaces lower than the top surfaces of the dielectric strip portions124, so that the sidewalls of the dielectric strip portions 124 have atleast some portions exposed. In accordance with alternative embodimentsof the present disclosure, the recessed STI regions 50 have their topsurfaces level with, higher than, or lower than the bottom surfaces ofthe dielectric strip portions 124. Throughout the description, theportions of the strips 142 and 242 higher than the top surfaces of theSTI regions 50 are referred to as fins (or protruding fins) 156 and 256.The fin 156 has a top end in a position higher than a top end of aninterface between the strip portion 234 b and the strip portion 236. Aninterface between the strip portion 124 and the strip portion 127 is ina position lower than a bottom end of the interface between the stripportion 234 b and the strip portion 236.

Referring to FIG. 10, a dummy gate stack 58 is formed on the topsurfaces and the sidewalls of (protruding) fins 156 and 256. It isappreciated that although one dummy gate stack 58 is illustrated forclarity, there may be a plurality of dummy gate stacks formed, which areparallel to each other, with the plurality of dummy gate stacks crossingthe same semiconductor fin(s) 156 and 256. The dummy gate stack 58 mayinclude a dummy gate dielectric 60 and a dummy gate electrode 62 overthe dummy gate dielectric 60. The dummy gate electrode 62 may be formed,for example, using polysilicon, and other materials may also be used.The dummy gate stack 58 may also include one (or a plurality of) hardmask layer 64 over the dummy gate electrode 62. The hard mask layer 64may be formed of silicon nitride, silicon carbo-nitride, or the like.The dummy gate stack 58 may cross over a single one or a plurality ofprotruding fins 156 and 256 and/or STI regions 50. The dummy gate stack58 also has a lengthwise direction perpendicular to the lengthwisedirections of protruding fins 156 and 256.

Next, referring to FIG. 11, a spacer layer 66 is deposited. Inaccordance with some embodiments of the present disclosure, the spacerlayer 66 is formed of a dielectric material such as silicon nitride,silicon carbon-oxyitride (SiCN), or the like, and may have asingle-layer structure or a multi-layer structure including a pluralityof dielectric layers. The formation may be performed through a conformaldeposition method such as ALD or CVD.

FIG. 12 illustrates the etching of the spacer layer 66, resulting in theformation of gate spacers 68 on the sidewalls of the dummy gate stack58. The etching is performed anisotropically, so that the portions ofthe spacer layer 66 on protruding fins 156 and 256 are removed.

Next, epitaxy regions 172 and 272 are formed by selectively growingsemiconductor materials on protruding fins 156 and 256, respectively,resulting in the structure in FIG. 13. Epitaxy regions 172 and 272 areepitaxially grown in different epitaxy processes, with each includingforming a mask layer (not shown) on one of the epitaxy regions 172 and272, so that the epitaxy regions may be formed on the other one ofepitaxy regions 172 and 272. Depending on whether the resulting FinFETis a p-type FinFET or an n-type FinFET, a p-type or an n-type impuritymay be in-situ doped with the proceeding of the epitaxy. For example,epitaxy regions 172 in the n-type device regions 100 may be formed ofsilicon phosphorous (SiP) or silicon carbon phosphorous (SiCP), andepitaxy regions 272 in the p-type region 200 may be formed of silicongermanium boron (SiGeB).

After the epitaxy step, epitaxy regions 172 and strip portions 127 maybe further implanted with an n-type impurity to form source and drainregions 174 for the n-type FinFET. Epitaxy regions 272 and stripportions 234 a, 234 b and 236 may also be implanted with a p-typeimpurity to form source and drain regions 274 for the p-type FinFET. Inaccordance with alternative embodiments of the present disclosure, theimplantation step is skipped when epitaxy regions 172 and 272 arein-situ doped with the p-type or n-type impurity during the epitaxy.

Although FIG. 13 illustrates that the source/drain regions 174 areseparated from each other, and the source/drain regions 274 areseparated from each other, it is realized that depending on how long theepitaxy processes last, the source/drain regions 174 may be merged witheach other or remain separated from each other, and the source/drainregions 274 may be merged with each other or remain separated from eachother. Also, the shapes of the epitaxy regions 172 and 272 may besimilar to what is shown, or have other shapes such as spade/diamondshapes.

FIG. 14 illustrates a perspective view of the structure with a contactetch stop layer (CESL) 76 and an Inter-Layer Dielectric (ILD) 78 beingformed. The CESL 76 may be formed of silicon nitride, siliconcarbo-nitride, or the like. The CESL 76 may be formed using a conformaldeposition method such as ALD, for example. The ILD 78 may include adielectric material formed using, for example, FCVD, spin-on coating,CVD, or other deposition methods. ILD 78 may also be formed of TetraEthyl Ortho Silicate (TEOS) oxide, Plasma Enhanced CVD (PECVD) oxide(SiO₂), Phospho-Silicate Glass (PSG), Boro-Silicate Glass (BSG),Boron-Doped Phospho-Silicate Glass (BPSG), or the like. A planarizationstep such as CMP or mechanical grinding may be performed to level thetop surfaces of the ILD 78, the dummy gate stack 58, and the gatespacers 68 with each other.

Next, the dummy gate stack 58, which include hard mask layer 64, thedummy gate electrode 62 and the dummy gate dielectric 60, is replacedwith replacement gate stack 84, which include metal gates 82 andreplacement gate dielectrics 80 as shown in FIG. 15. In the removal ofdummy gate stacks 58, dielectric strip portions 124 (FIG. 9) that werepreviously buried under the dummy gate stacks are exposed, and are atleast recessed laterally due to the similarity of its material to thematerial of the dummy gate dielectric. In accordance with someembodiments of the present disclosure, an additional etching process,which may be a wet etching process, is further performed after theremoval of the dummy gate stack if the material of strip portions 124 isdifferent from that of STI regions 50, so that strip portions 124 areremoved without damaging STI regions 50.

When replacing gate stacks, hard mask layers 64, dummy gate electrodes62 and dummy gate dielectrics 60 (FIG. 14) are first removed in one or aplurality of etching steps, resulting in a trench (opening) to be formedbetween gate spacers 68. In the formation of the replacement gates, agate dielectric layer 80 (FIG. 15) is first formed, which extends intothe trench left by the removed dummy gate stack, and may have a portionextending over ILD 78. In accordance with some embodiments of thepresent disclosure, the gate dielectric 80 includes an interfacial layer(IL, not shown separately) as its lower part. The IL may include anoxide layer such as a silicon oxide layer, which is formed through achemical oxidation process or a deposition process. The gate dielectric80 may also include a high-k dielectric layer formed over the IL. Thehigh-k dielectric layer is formed as a conformal layer, and includes ahigh-k dielectric material such as hafnium oxide, lanthanum oxide,aluminum oxide, zirconium oxide, or the like. The dielectric constant(k-value) of the high-k dielectric material is higher than 3.9, and maybe higher than about 7.0. In accordance with some embodiments of thepresent disclosure, the high-k dielectric layer in gate dielectric 80 isformed using ALD or CVD.

The fate electrode 82 is formed over the gate dielectric 80 and fillingthe remaining portion of the trench. The formation of the gate electrode82 may include a plurality of deposition processes to deposit aplurality of conductive layers, and performing a planarization step toremove the excess portions of the conductive layers over the ILD 78. Thedeposition of the conductive layers may be performed using conformaldeposition methods such as ALD or CVD.

The gate electrode 82 may include a diffusion barrier layer and one (ormore) work-function layer over the diffusion barrier layer. Thediffusion barrier layer may be formed of titanium nitride (TiN), whichmay (or may not) be doped with silicon to form TiSiN. The work-functionlayer determines the work function of the gate, and includes at leastone layer, or a plurality of layers formed of different materials. Thespecific material of the work-function layer is selected according towhether the respective FinFET is an n-type FinFET or a p-type FinFET.For example, for the n-type FinFET in device regions 100, thework-function layer may include a TaN layer and a titanium aluminum(TiAl) layer over the TaN layer. For the p-type FinFET in device region200, the work-function layer may include a TaN layer, a TiN layer overthe TaN layer, and a TiAl layer over the TiN layer. After the depositionof the work-function layer(s), another barrier layer, which may beanother TiN layer, is formed. The gate electrode 82 may also include afilling metal, which may be formed of tungsten or cobalt, for example.After the formation of the replacement gate stack 84, the replacementgate stack 84 is etched back, and dielectric hard mask 86 is formed overthe etched-back replacement gate 84.

After the structure as shown in FIG. 15 is formed, the ILD 78 and theCESL 76 are etched to form contact openings. The etching may beperformed using, for example, Reactive Ion Etch (RIE). In a subsequentstep, as shown in FIG. 16, source/drain contact plugs 88 are formed inthe contact openings. Before forming the contact plugs 88, the portionsof CESL 76 exposed to the contact opens are first etched, revealingepitaxy regions 172 and 272. Silicide regions 90 are then formed on theepitaxy regions 172 and 272. In accordance with some embodiments of thepresent disclosure, the contact plugs 88 include barrier layers and ametal-containing material over the respective barrier layers. Inaccordance with some embodiments of the present disclosure, theformation of the contact plugs 88 includes forming a blanket barrierlayer and a metal-containing material over the blanket barrier layer,and performing a planarization to remove excess portions of the blanketbarrier layer and the metal-containing material. The barrier layer maybe formed of a metal nitride such as titanium nitride or tantalumnitride. The metal-containing material may be formed of tungsten,cobalt, copper, or the like. N-type FinFETs 192 and a p-type FinFET 292are thus formed.

FIGS. 17 through 32 illustrate the cross-sectional views and perspectiveviews of intermediate stages in the formation of the hybrid substrateand the FinFETs in accordance with some embodiments of the presentdisclosure. FIGS. 17-32 show substantially the same structures as FIGS.1-16, except there is a larger p-type device region 200′ than the p-typedevice region 200 as shown in FIGS. 1-16.

Referring to FIG. 17, a hybrid substrate 20 is provided. The hybridsubstrate 20 includes p-type well regions 100 and an n-type well region200′ between the p-type well regions 100. The p-type well regions 100and the n-type well region 200′ are formed, for example, throughimplantations. The n-type well region 200′ has a width greater than thatof the n-type well region 200 as shown in FIG. 1. The p-type wellregions 100 can be equivalently referred to as n-type device regions,and the n-type well region 200′ can be equivalently referred to as ap-type device region in some embodiments. The hybrid substrate 20includes a crystalline silicon layer 22, a dielectric layer 24 over thesilicon layer 22, and a crystalline silicon layer 26 over dielectriclayer 24. The silicon layer 22, the dielectric layer 24 and the siliconlayer 26 are discussed previously with respect to FIG. 1, and detaileddescription thereof is thus not repeated for the sake of brevity.

Referring to FIG. 18, an epitaxy is performed to grow silicon layer 28on silicon layer 26. Epitaxy growth of the silicon layer 28 is discussedpreviously with respect to FIG. 2, and the detailed description thereofis thus not repeated for the sake of brevity.

FIG. 19 illustrates the recessing of silicon layer 28 and hybridsubstrate 20 in the p-type device region 200′, and the recessing is notperformed in the n-type device regions 100. Recess 35′ is thus formed inthe p-type device region 200′. The recess 35′ has a width greater than awidth of the recess 35 as shown in FIG. 3 because of the larger p-typedevice region 200′. Formation of the recess 35′ is discussed previouslywith respect to formation of the recess 35 as shown in FIG. 3, and thedetailed description therefor is thus not repeated for the sake ofbrevity.

Next, a spacer layer is deposited, followed by an anisotropic etch toremove the horizontal portions of the spacer layer, so that a spacer 32is formed. Formation of the spacer 32 is discussed previously withrespect to FIG. 3, and the detailed description thereof is thus notrepeated for the sake of brevity.

After the spacer 32 is formed, a first epitaxy process and a secondepitaxy process are performed to form first and second epitaxy layers34′ and 36′, respectively, and the resulting structure is shown in FIGS.20A and 20B. FIG. 20A illustrates a perspective view of the structure,and FIG. 20B illustrates a cross-sectional view along line B-B in FIG.20A. Epitaxy growth of the first epitaxy layer 34′ is substantially thesame as epitaxy growth of the first epitaxy layer 34 as shown in FIGS.4A and 4B, except that the first portion 34 a′ of the first epitaxylayer 34 has larger (100) surface than the first portion 34 a of thefirst epitaxy layer 34. This is because the first epitaxy layer 34′ isgrown on a larger p-type device region 200′. Epitaxy growth of thesecond epitaxy layer 36′ is substantially the same as epitaxy growth ofthe second epitaxy layer 36 as shown in FIGS. 4A and 4B, except that thefirst portion 36 a′ of the second epitaxy layer 36 is larger than thefirst portion 36 a of the second epitaxy layer 36. This is because thesecond epitaxy layer 36′ is grown on a larger top surface of the firstportion 34 a′ that extends along the (100) plane. Similar to the firstand second epitaxy layers 34 and 36, the first epitaxy layer 34′includes a second portion 34 b′ grown on a sidewall of the first portion34 a′ that extends along the (111) plane and having a greater germaniumconcentration than that of the first portion 34 a′, and the secondepitaxy layer 36′ includes a second portion 36 b′ grown on a surface ofthe second portion 34 b′ of the first epitaxy layer 34′ that extendsalong the (111) plane and having a greater germanium concentration thanthat of the first portion 36 a′.

In some embodiments, the germanium concentration of the second portion34 b′ and the germanium concentration of the first portion 34 a′ have adifference greater than about 5%. For example, the germaniumconcentration of the first portion 34 a′ is in a range from about 20% toabout 35%, and the germanium concentration of the second portion 34 b′is in a range from about 25% to about 40%. If the epitaxy process of thefirst SiGe layer 34 continues until the recess 35 is filled with thesecond portion 34 b, the resulting fins (See FIG. 23) would include alarge region (i.e., large second portion 34 b′) having unwantedlyincreased germanium concentration and a small region (i.e., small firstportion 34 a′ compared to the large second portion 34 b′) having thedesired germanium concentration. For example, a volume percentage of theportion having unwantedly increased germanium concentration in theresulting fin might be from a range about 31% to about 37%, which inturn aggravates non-uniformity of germanium concentration in theresulting fins.

As a result, epitaxy growth of the first SiGe layer 34′ stops as long asthe epitaxy growth on the (111) plane occurs. Afterwards, processconditions for the SiGe epitaxy process are changed to epitaxially growthe second SiGe layer 36′ on the first SiGe layer 34′. Less germanium isintroduced in the formation of the second SiGe layer 36′ than in theformation of the first SiGe layer 34. In this way, the resulting secondSiGe layer 36′ has a lower germanium atomic percentage than the secondportion 34 b′ of the first SiGe layer 34′. As a result, thenon-uniformity of the germanium concentration in the resulting fins ismitigated in spite of the unwantedly high germanium concentration occursin the second portion 34 b′ of the first SiGe layer 34′. For example, avolume percentage of the portion having unwantedly increased germaniumconcentration (i.e., second portion 34 b′ of the first SiGe layer 34′)in the resulting fin might be from a range about 4% to about 8%, whichin turn significantly reduce non-uniformity of germanium concentrationin the resulting fins. For example, the germanium concentration of thefirst portion 36 a′ of the second SiGe layer 36′ is in range from about15% to about 30%, and the germanium concentration of the second portion36 b′ of the second SiGe layer 36′ is in a range from about 20% to about35%.

Formation of the second SiGe layer 36′ may be performed in-situ with theformation of the first SiGe layer 34′. In some exemplary embodiments,the partial pressures, hence flow rates of the germanium-containinggases such as GeH₄ are reduced to reduce the germanium concentration inthe second SiGe layer 36′. In some embodiments, the GeH₄ partialpressure difference between growing the first SiGe layer 34′ and growingthe second SiGe layer 36′ is in a range from about 30 mtorr to about 150mtorr. If the GeH₄ partial pressure difference between growing the firstSiGe layer 34′ and growing the second SiGe layer 36′ is greater thanabout 150 mtorr, the germanium concentration difference between thesecond portion 36 b′ of the second SiGe layer 36′ and the first portion34 a′ of the first SiGe layer 34′ might be unsatisfactory for improvingthe germanium concentration uniformity in the resulting fins. If theGeH₄ partial pressure difference between growing the first SiGe layer34′ and growing the second SiGe layer 36′ is less than about 30 mtorr,the germanium concentration difference between the second portion 36 b′of the second SiGe layer 36′ and the first portion 34 a′ of the firstSiGe layer 34′ might be unsatisfactory for improving the germaniumconcentration uniformity in the resulting fins. In some embodiments, thefirst and second epitaxy layers 34′ and 36′ are in combination referredto as an multi-layer epitaxy structure 37′.

Moreover, misfit dislocations might occur at the interface between thesilicon layer 22 and the SiGe structure 37′, if the thickness of theSiGe structure 37′ is greater than a critical thickness. However, thecritical thickness associated with the misfit dislocations is inpositive correlation of the average germanium percentage in the SiGestructure 37′. Because the SiGe structure 37′ includes reduced averagegermanium concentration as discussed above, the critical thickness ofthe SiGe structure 37′ can be increased, which in turn will result inreduced misfit dislocations. In some embodiments, the first SiGe layer34′ has a thickness T3 in a range from about 50 nm to about 70 nm. Ifthe thickness T3 of the first SiGe layer 34′ is greater than about 70nm, misfit dislocations might be increased. In some embodiments, thesecond SiGe layer 36′ has a thickness T4 in a range from about 60 nm toabout 80 nm. If the thickness T4 of the second SiGe layer 36′ is greaterthan about 80 nm, misfit dislocations might be increased.

After the epitaxy of epitaxy layers 34′ and 36′, a planarization stepsuch as Chemical Mechanical Polish (CMP) or mechanical grinding isperformed to level the top surface of the structure 37, as shown inFIGS. 21A and 21B. FIG. 21A illustrates a perspective view of thestructure, and FIG. 21B illustrates a cross-sectional view along lineB-B as shown in FIG. 21A. The planarization is discussed previously withrespect to FIGS. 5A and 5B, and the detailed description regarding theplanarization is thus not repeated for the sake of brevity.

Referring to FIG. 22, a protection layer 38 is formed. In accordancewith some embodiments of the present disclosure, the protection layer 38is formed of silicon, and is deposited on the top surface the structureshown in FIG. 21A. The protection layer 38 is also free from germanium.The deposition may be achieved through an epitaxy process, so that thesilicon layer is a crystalline layer.

The following steps as shown in FIGS. 22 and 23 illustrate the formationof semiconductor strips. The strips may be patterned by any suitablemethod, as discussed previously with respect to FIGS. 6 and 7. Inaccordance with some exemplary embodiments as shown in FIG. 22, masklayer(s) are deposited over the protection layer 38, and are thenpatterned to form masks 40, which are used as etching mask for formingsemiconductor strips. In accordance with some embodiments of the presentdisclosure, masks 40 include a plurality of layers formed of differentmaterials. For example, masks 40 may include layers 40A formed ofsilicon oxide, and layers 40B over the respective layers 40, with layers40B formed of silicon nitride.

FIG. 23 illustrates a perspective view of the structure after the stripformation. Referring to FIG. 23, an etching process is performed to etchthe substrate and epitaxy layers, so that strips 142 are formed inn-type device regions 100, and strips 242′ and 244′ are formed in p-typedevice region 200, respectively. Recesses 44 are formed to separatestrips 142 and 242′ and 244′. Strips 142 include portions 122, 124, and127. Strip portions 122 are the remaining portions of the patternedsilicon layer 22. Strip portions 124 are the remaining portions ofdielectric layer 24 (FIG. 22). Strip portions 127 are the remainingportions of silicon layers 26 and 28 (FIG. 22). In accordance with someembodiments of the present disclosure, silicon layers 26 and 28 are(100) R45 layers. Accordingly, both the top surface and sidewalls ofstrips 142 have (100) surface orientations. Strips 242 include portions222 and 234 a′. Strip portions 222 are the remaining portions of thepatterned silicon layer 22. Strip portions 234 a′ are the remainingportions of first portion 34 a′ of the first epitaxy layer 34′ (FIG.22). Strips 244′ include portions 222, 234 a′, 234 b′ and 236′. Stripportions 234 b′ are remaining portions of the second portion 34 b′ ofthe first epitaxy layer 34′ (FIG. 22). Strip portions 236 b′ areremaining portions of the second portion 36 b′ of the second epitaxylayer 36′ (FIG. 22). Strip portions 234 a′ and 236 b′ are SiGe portionsand have substantially the same germanium concentration. Strip portions234 b′ are SiGe portions having higher germanium concentrations thanstrip portions 234 a′ and 236 b′. In some embodiments, a germaniumconcentration difference between the strip portions 234 a′ and 234 b′ isgreater than a germanium concentration difference between the stripportions 234 a′ and 236 b′, and a germanium concentration differencebetween the strip portions 234 b′ and 236 b′ is greater than thegermanium concentration difference between the strip portions 234 a′ and236 b′ as well.

FIG. 24 illustrates the formation of a liner 48, which is used to maskthe sidewalls of strip portions 234 a′, 234 b′ and 236 b′ fromoxidation. Formation of the liner 48 is discussed previously withrespect to FIG. 8, and the detailed description thereof is thus notrepeated for the sake of brevity.

FIG. 25 illustrates the formation of STI regions 50, which includesdielectric liners 52 and dielectric regions 54 over dielectric liners52. Formation of the STI regions 50 is discussed previously with respectto FIG. 9, and the detailed description thereof is thus not repeated forthe sake of brevity.

Next, a planarization such as CMP or mechanical grinding is performed ondielectric regions 54 and dielectric liners 52. The planarization may beperformed using masks 40 (FIG. 24) as a stop layer. Next, masks 40 areremoved, followed by recessing dielectric regions 54 and dielectricliners 52. The resulting structure is shown in FIG. 25. The remainingportions of dielectric regions 54 and dielectric liners 52 are referredto as STI regions 50. In accordance with some embodiments of the presentdisclosure, the recessing is performed until the recessed STI regions 50have their top surfaces lower than the top surfaces of dielectric stripportions 124, so that the sidewalls of dielectric strip portions 124have at least some portions exposed. In accordance with alternativeembodiments of the present disclosure, the recessed STI regions 50 havetheir top surfaces level with, higher than, or lower than the bottomsurfaces of dielectric strip portions 124. Throughout the description,the portions of strips 142, 242′ and 244′ higher than the top surfacesof STI regions 50 are referred to as fins (or protruding fins) 156, 256′and 258′. The fin 156 has a top end in a position higher than a top endof an interface between the strip portion 234 b′ and the strip portion236 b′. An interface between the strip portion 124 and the strip portion127 is in a position lower than a bottom end of the interface betweenthe strip portion 234 b′ and the strip portion 236 b′.

Referring to FIG. 26, a dummy gate stack 58 is formed on the topsurfaces and the sidewalls of (protruding) fins 156′, 256′ and 258′.Formation of the dummy gate stack 58 is discussed previously withrespect to FIG. 10, and the detailed description thereof is thus notrepeated for the sake of brevity.

Next, referring to FIG. 27, a spacer layer 66 is deposited. Inaccordance with some embodiments of the present disclosure, the spacerlayer 66 is formed of a dielectric material such as silicon nitride,silicon carbon-oxyitride (SiCN), or the like, and may have asingle-layer structure or a multi-layer structure including a pluralityof dielectric layers. The formation may be performed through a conformaldeposition method such as ALD or CVD.

FIG. 28 illustrates the etching of the spacer layer 66, resulting in theformation of gate spacers 68 on the sidewalls of the dummy gate stack58. Etching the spacer layer 66 is discussed previously with respect toFIG. 12, and the detailed description thereof is thus not repeated forthe sake of brevity.

Next, epitaxy regions 172 are formed by selectively growingsemiconductor materials on protruding fins 156, and epitaxy regions 272are formed by selectively growing semiconductor materials on protrudingfins 256′ and 258′, resulting in the structure in FIG. 29. Epitaxialgrowth of the pitaxy regions 172 and 272 is discussed previously withrespect to FIG. 13, and the detailed description thereof is thus notrepeated for the sake of brevity.

After the epitaxy step, epitaxy regions 172 and strip portions 127 maybe further implanted with an n-type impurity to form source and drainregions 174 for the n-type FinFETs. Epitaxy regions 272 and stripportions 234 a′ of strips 242′ and strip portions 234 a′, 234 b′ and 236b′ of strips 244′ may also be implanted with a p-type impurity to formsource and drain regions 274 for the p-type FinFETs. In accordance withalternative embodiments of the present disclosure, the implantation stepis skipped when epitaxy regions 172 and 272 are in-situ doped with thep-type or n-type impurity during the epitaxy.

Although FIG. 29 illustrates that source/drain regions 174 are separatedfrom each other, and source/drain regions 274 are separated from eachother, it is realized that depending on how long the epitaxy processeslast, source/drain regions 174 may be merged with each other or remainseparated from each other, and source/drain regions 274 may be mergedwith each other or remain separated from each other. Also, the shapes ofthe epitaxy regions 172 and 272 may be similar to what is shown, or haveother shapes such as spade/diamond shapes. Air gaps may be formeddirectly underlying the merged portions of epitaxy regions 172, and/ordirectly underlying the merged portions of epitaxy regions 272.

FIG. 30 illustrates a perspective view of the structure with ContactEtch Stop Layer (CESL) 76 and Inter-Layer Dielectric (ILD) 78 beingformed. Formation of the CESL 76 and the ILD 78 is discussed previouslywith respect to FIG. 14, and the detailed description thereof is thusnot repeated for the sake of brevity.

Next, dummy gate stack 58, which include hard mask layer 64, dummy gateelectrode 62 and dummy gate dielectric 60, is replaced with replacementgate stack 84, which include metal gates 82 and replacement gatedielectrics 80 as shown in FIG. 31. The process can be referred to as agate last process or gate replacement process, which is discussedpreviously with respect to FIG. 25 and is thus not repeated for the sakeof brevity. After the formation of the replacement gate stack 84,replacement gate stack 84 is recessed, and dielectric hard mask 86 isfilled into the recess.

After the structure as shown in FIG. 31 is formed, ILD 78 and CESL 76are etched to form contact openings. The etching may be performed using,for example, Reactive Ion Etch (RIE). In a subsequent step, as shown inFIG. 32, source/drain contact plugs 88 are formed. Formation of thesource/drain contact plugs 88 is discussed previously with respect toFIG. 16, and detailed description thereof is thus not repeated for thesake of brevity. N-type FinFETs 192 and a p-type FinFET 292′ are thusformed.

Based on the above discussion, it can be seen that the presentdisclosure offers advantages. It is understood, however, that otherembodiments may offer additional advantages, and not all advantages arenecessarily disclosed herein, and that no particular advantages isrequired for all embodiments. One advantage is that uniformity of thegermanium concentration in the SiGe fin can be improved in spite of aportion of the SiGe fin grown on (111) plane has higher germaniumconcentration than other portions of the SiGe fin. This is because thelater-grown SiGe layer is formed using a germanium-containing gas with areduced flow rate compared to that of forming the initially-grown SiGelayer. Another advantage is that misfit dislocations can be reducedbecause the associated critical thickness of the SiGe fin is increasedby reduction of the average germanium concentration in the SiGe fin.

In some embodiments, a semiconductor device includes a substrate, afirst semiconductor fin, and a gate stack. The first semiconductor finis over the substrate and includes a first germanium-containing layerand a second germanium-containing layer over the firstgermanium-containing layer. The first germanium-containing layer has agermanium atomic percentage higher than a germanium atomic percentage ofthe second germanium-containing layer. The gate stack is across thefirst semiconductor fin.

In some embodiments, a semiconductor device includes a substrate, afirst semiconductor fin, and a p-type epitaxy region. The firstsemiconductor fin protrudes above the substrate. The first semiconductorfin includes a first germanium-containing layer and a secondgermanium-containing layer having a germanium atomic percentage lowerthan a germanium atomic percentage of the first germanium-containinglayer. The p-type epitaxy region is across the first semiconductor fin.The second germanium-containing layer is between the p-type epitaxyregion and the first germanium-containing layer.

In some embodiments, a method includes etching a hybrid substrate toform a recess extending into the hybrid substrate, the hybrid substrateincluding a first semiconductor layer having a first surfaceorientation, a dielectric layer over the first semiconductor layer, anda second semiconductor layer having a second surface orientationdifferent from the first surface orientation, after the etching, a topsurface of the first semiconductor layer being exposed to the recess;forming a spacer on a sidewall of the recess; performing a first epitaxyprocess to grow a first germanium-containing layer from the exposed topsurface of the first semiconductor layer; performing a second epitaxyprocess to grow a second germanium-containing layer from the firstgermanium-containing layer, a flow rate of a germanium-containing gasused in the second epitaxy process being less than a flow rate of agermanium-containing gas used in the first epitaxy process; andpatterning the first and second germanium-containing layers to form afin.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor device, comprising: a substrate;a first semiconductor fin over the substrate and comprising a firstgermanium-containing layer and a second germanium-containing layer overthe first germanium-containing layer, wherein the firstgermanium-containing layer has a germanium atomic percentage higher thana germanium atomic percentage of the second germanium-containing layer;and a gate stack across the first semiconductor fin.
 2. Thesemiconductor device of claim 1, wherein the first semiconductor finfurther comprises a third germanium-containing layer under the firstgermanium-containing layer and having a germanium atomic percentagelower than the germanium atomic percentage of the firstgermanium-containing layer.
 3. The semiconductor device of claim 2,wherein a germanium atomic percentage difference between the first andsecond germanium-containing layers is greater than a germanium atomicpercentage difference between the second and third germanium-containinglayers.
 4. The semiconductor device of claim 2, wherein a germaniumatomic percentage difference between the first and thirdgermanium-containing layers is greater than a germanium atomicpercentage difference between the second and third germanium-containinglayers.
 5. The semiconductor device of claim 2, wherein the germaniumatomic percentage of the second germanium-containing layer issubstantially the same as the germanium atomic percentage of the thirdgermanium-containing layer.
 6. The semiconductor device of claim 2,wherein an interface between the first and third germanium-containinglayers is on a (111) plane.
 7. The semiconductor device of claim 1,wherein an interface between the first and second germanium-containinglayers is on a (111) plane.
 8. The semiconductor device of claim 1,further comprising: a second semiconductor fin over the substrate,wherein the second semiconductor fin has a top end in a position higherthan a top end of an interface between the first and secondgermanium-containing layers.
 9. The semiconductor device of claim 8,further comprising: a dielectric layer between the second semiconductorfin and the substrate.
 10. The semiconductor device of claim 9, whereinan interface between the dielectric layer and the second semiconductorfin is in a position lower than a bottom end of the interface betweenthe first and second germanium-containing layers.
 11. A semiconductordevice, comprising: a substrate; a first semiconductor fin protrudingabove the substrate, the first semiconductor fin comprising a firstgermanium-containing layer and a second germanium-containing layerhaving a germanium atomic percentage lower than a germanium atomicpercentage of the first germanium-containing layer; and a p-type epitaxyregion across the first semiconductor fin, the secondgermanium-containing layer being between the p-type epitaxy region andthe first germanium-containing layer.
 12. The semiconductor device ofclaim 11, further comprising: a second semiconductor fin protrudingabove the substrate and having a germanium atomic percentage lower thanthe germanium atomic percentage of the first germanium-containing layerof the first semiconductor fin.
 13. The semiconductor device of claim12, wherein the germanium atomic percentage of the second semiconductorfin is substantially the same as the germanium atomic percentage of thesecond germanium-containing layer of the first semiconductor fin. 14.The semiconductor device of claim 12, further comprising: a thirdsemiconductor fin protruding above the substrate and free of germanium.15. The semiconductor device of claim 11, wherein the p-type epitaxyregion is interfaced with the first and second germanium-containinglayers.
 16. The semiconductor device of claim 11, wherein the firstsemiconductor fin further comprises a third germanium-containing layerbetween the first germanium-containing layer and the substrate, and thethird germanium-containing layer has a germanium atomic percentage lowerthan the germanium atomic percentage of the first germanium-containinglayer.
 17. The semiconductor device of claim 16, wherein the germaniumatomic percentage of the second germanium-containing layer issubstantially the same as the germanium atomic percentage of the thirdgermanium-containing layer.
 18. A method, comprising: etching a hybridsubstrate to form a recess extending into the hybrid substrate, whereinthe hybrid substrate comprises a first semiconductor layer having afirst surface orientation, a dielectric layer over the firstsemiconductor layer, and a second semiconductor layer having a secondsurface orientation different from the first surface orientation,wherein after the etching, a top surface of the first semiconductorlayer is exposed to the recess; forming a spacer on a sidewall of therecess; performing a first epitaxy process to grow a firstgermanium-containing layer from the exposed top surface of the firstsemiconductor layer; performing a second epitaxy process to grow asecond germanium-containing layer from the first germanium-containinglayer, wherein a flow rate of a germanium-containing gas used in thesecond epitaxy process is less than a flow rate of agermanium-containing gas used in the first epitaxy process; andpatterning the first and second germanium-containing layers to form afin.
 19. The method of claim 18, wherein the first epitaxy process isperformed such that the first germanium-containing layer comprises afirst portion grown from the first semiconductor layer and a secondportion grown from a facet of the first portion, the facet is on a (111)plane, and the second portion has a germanium atomic percentage higherthan a germanium atomic percentage of the first portion.
 20. The methodof claim 19, wherein the second epitaxy process is performed such thatat least a portion of the second germanium-containing layer is grownfrom the second portion.